U.S. Pat. No. 4,287,404 illustrates a typical EDM apparatus as well as an electrode suitable for use in an EDM process. In its simplest terms, the EDM process uses an electrode that is brought into close proximity to a work piece. An electrical potential is applied between the electrode and the work piece at a level sufficient to cause an electrical discharge which causes erosion of both the electrode and the work piece.
One type of commercial EDM equipment uses a travelling wire electrode so that fresh electrode material is always present thus enabling a constant cutting action. In this travelling wire EDM process, a traveling wire moves past the material to be cut and a pulsed potential is applied between the wire electrode and the workpiece. A spark is generated, the heat of which melts a small volume of the workpiece (and electrode). A dielectric, usually deionized water is used to flush the cutting region and removes the eroded material. This process, spark...melt...flush, is repeated thousands of times per second.
The efficiency at which the cutting action takes place is in large measure dependent on the construction and material properties of the wire electrode. The electrode must be capable of sustaining high current densities and therefore must be electrically conductive and must also have relatively high mechanical strength since it must be kept under tension as it moves past the workpiece in order for the cutting action to maintain precise tolerances.
In general, wire materials that have high mechanical strength are generally poor conductors of electrical current. For this reason, various hybrid or composite wires have been suggested in the past. In the case of U.S. Pat. No. 4,287,404, referenced above, an electrode was disclosed that included an outer coating of either zinc, cadmium, tin, lead, antimony or bismuth, or allies thereof plated onto various metallic substrates including a copper clad steel core.
In U.S. Pat. No. 4,686,153 a composite electrode was disclosed which included a copper clad steel wire that was plated with zinc using an electroplating or hot galvanizing process. Following the plating process, the wire was heated in order to disperse copper into the zinc layer to form a gradient layer in which the concentration of zinc decreased in the radial direction as the center of the electrode was approached. It is believed that the wire disclosed in this patent is very expensive and as a result was not economically feasible. In addition, it is believed that a zinc gradient in which the zinc concentration decreases as one moves radially inwardly from the outer surface, results in a wire construction that is not efficient and may perform erratically since the concentration of zinc (which improves the flushability) varies and in fact is reduced as the center of the wire is approached during erosion.